This invention relates to a method and apparatus for forming an absorbent core having a multiplicity of components. More particularly, the present invention relates to a method and apparatus for producing a homogeneous distribution of super absorbent material throughout an absorbent core.
Absorbent articles including bandages, disposable diapers, and sanitary napkins, generally include an absorbent core that has a multiplicity of components so as to improve the article's absorption and retention characteristics. These absorbent cores have had their total absorbency improved greatly by the addition of super absorbent material to the commonly used absorbent fibrous materials.
Typically, the absorbent fibrous material is composed of cellulose wadding or cellulosic wood pulp material commonly referred to as “fluff”, although a mixture of natural and synthetic fibers is within the scope of the invention. An absorbent core composed of wood pulp fluff is typically formed by employing conventional air laying techniques.
The ability of these absorbent cores to manage the typical surges of liquid flow is heavily dependent on the proper distribution of super absorbent material within the absorbent fluff. When most super absorbent materials absorb aqueous fluids, they swell substantially, often to double their dry dimensions or more at saturation. As these super absorbent materials absorb fluid and swell, they generally become a gelatinous mass. If the super absorbent material is in a particulate form and the particles are close to one another, they can create a phenomenon known as “gel-blocking,” wherein the saturated blocks of super absorbent material inappropriately provide a barrier to additional surges of liquid.
Thus, for maximum effectiveness of the super absorbent material, absorbent structures which include such materials in particulate form preferably maintain separation of the particles from one another to permit maximum absorption and swelling without allowing the particles to coalesce and form the gel barrier.
To avoid this barrier and to optimize performance, various modes of distribution of super absorbent material throughout the absorbent core have been tried. Layering and blending the super absorbent material within the absorbent core make up the principle two modes. Examples of generally layered applications can be seen in U.S. Pat. No. 4,551,191 issued Nov. 5, 1985 to Kock et al. and U.S. Pat. No. 6,416,697 issued Jul. 9, 2002 to Venturino et al. In this method, super absorbent material is commonly distributed onto a moving porous web. Examples of blended applications can be seen in U.S. Pat. No. 4,764,325 issued Aug. 16, 1988 to Angstadt and U.S. Pat. No. 4,927,582 issued May 22, 1990 to Bryson. In a blended application, the super absorbent material first is mixed with the absorbent fluff. Then the mixture is laid down creating the absorbent structure.
Super absorbent material typically has the consistency of sand, while absorbent fluff is more fibrous, and tends to knit together. High concentrations of super absorbent material, if not distributed amongst the fluff fibers, performs poorly and tends to fall out of the product during manufacturing—a loss of investment.
Manufacturers have attempted to optimize distribution of the super absorbent material in order to get the most “bang for the buck” out of their investment. As well, optimizing the distribution and eliminating localized buildup of super absorbent material can improve the feel of the article on the user. For example, eliminating a localized buildup of super absorbent material along an inner layer of a diaper can eliminate the gritty feel of the super absorbent material next to the baby's skin, making for a much happier baby and a more effective diaper.
Likewise, there has been a trend in reducing the bulk of diapers, in attempts to make them more like underwear and to take up less shelf space in retailer's outlets. Generally, the thinner the diaper, the higher the concentration of super absorbent material required to produce the same level of absorbency. High levels of super absorbent material, however, tend to be more difficult to control and to maintain in position.
In solving these problems, the most friendly application of super absorbent material would seem to be a completely uniform (homogeneous) distribution, with no noticeable local concentrations of super absorbent material. To achieve homogenous blending, it is necessary to add the super absorbent material to the main air stream that carries the absorbent fluff to the deposition molds that form the cores.
Super absorbent material, however, has entirely different behavior from fluff fibers. It has a much higher density (thus a higher inertia) and does not easily follow the fluff fibers along their turbulent paths to their final, intended resting place. Once it has been imparted with a velocity, the super absorbent material tends to behave more like a bullet than a leaf in the wind. This all makes it difficult to mix the super absorbent material uniformly with the absorbent fluff. It is a problem of uniformly blending two very different components.
The prior art super absorbent distribution techniques such as those herein described earlier have made attempts in providing substantially uniform distribution of the particulate super absorbent material. However, none of the foregoing appears to have adequately understood and addressed the problems associated with the entirely different behaviors of the super absorbent material and the fluff fibers.
Because of the foregoing deficiencies in the art, an object of the present invention is to provide a method and apparatus that solves these problems by making the distribution of the super absorbent material within the fluff fibers more uniform. Homogeneous distribution can be achieved by essentially altering the behavior of the super absorbent material to make its behavior more closely replicate that of the absorbent fluff in which it must uniformly mix.